Catalytic radiant heater



CATALYTIC RADIANT HEATER Filed April 26, 1967 F \G. 2 F \G. 5

F-' IG. 6

INVENTOR.

CARL D. KEITH JOHN J. MOONEY ATTORNEY United States Patent O 3,441,359CATALYTIC RADIANT HEATER Carl D. Keith, Summit, and John J. Mooney,Wyckotf, N.J., assignors to Engelhard Industries, Inc, Newark, N.J., acorporation of Delaware Filed Apr. 26, 1967, Ser. No. 633,803 Int. Cl.F2311 13/12 US. Cl. 431-328 9 Claims ABSTRACT OF THE DISCLOSURE Thisdisclosure concerns heaters which utilize a fuel, generally gaseous innature, and an oxygen-containing gas such as air. The fuel and oxidizinggas are combined catalytically and burn without flame formation at thesurface of a non-combustible unitary ceramic skeletal block with gasflow channels. The devices are useful as heaters when the temperature ofthis surface is sufficiently high by virtue of the combustion to causeemission of substantial amounts of heat by radiation.

BACKGROUND OF THE INVENTION This invention pertains to new types ofdevices for emitting heat by radiation. More particularly, it relates toheaters comprising precious metal catalysts which produce flamelesscombustion and provide radiant infrared energy. These heaters useunitary channeled ceramic blocks to assist in the oxidation. Thesedevices are useful for removing paint, varnish and the like, installingand removing asphalt tile, baking and drying painted surfaces, heatingand radiating greenhouses, and other purposes.

Catalytic heaters which utilize a precious metal catalyst such as aplatinum black on substrate such as asbes os fiber or cloth are known.Such heaters are generally bulky, depend on secondary air forcombustion, and require a large flame for ignition. Moreover, thecombustion gases tend to flow unevenly to the surface of such heaters.Uneven burning and hot spots result causing a decrease in the catalystlife. This uneven burning also produces unoxidized or partially oxidizedcombustion products thereby providing inefficient operation andpoisonous carbon monoxide emission. Other infrared energy producingheaters which use ceramic gas flow channels are known, but these heatersare not capable of using a broad range of air-fuel mixtures. Nor arethey capable of providing a large turn-down ratio. Turn-down ratiorefers to ability to decrease the rate of fuel consumption by decreasingfuel flow. This reduction automatically reduces the air input. Aturn-down ratio of 100:1 means a fuel may be consumed at a maximum ratein a burner as well as this maximum rate. This turn-down feature is verydesirable when the catalytic heater is called upon to perform variousjobs, each requiring quite different heating requirements.

SUMMARY OF THE INVENTION The present invention pertains to catalyticradiant heaters which use simple venturi means to provide air forcombustion and assist in distributing the combustion mixture, and aunitary ceramic block support with multiple gas passages, on the surfaceof which are deposited a platinum group metal and preferably arefractory metal oxide. The catalytic support assists in catalyzing theoxidation of the fuel, preventing combustion flashback, and radiatingheat to the object to be heated.

It is an object of this invention to provide a catalytic heater whichprovides for complete combustion of fuel. It is a further object toprovide a device which prevents 3,441,359 Patented Apr. 29, 1969 loss ofcatalyst life due to creation of hot spots during operation. A stillfurther object is provision of a catalytic heater simple in design whichprovides a broad range of usable air-fuel ratios as well as a largeturn-down ratio and variation in B.t.u. availability. Other objects willbe apparent from the following detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS FIGURE 1 is an elevational view of ahousing of a catalytic heater of this invention.

FIGURE 2 represents a plan view of a catalyst block, showing anembodiment which is shaped to be encased in the face of the housing ofFIGURE 1, where the face of the housing is in an elliptical shape.

FIGURE 3 is a plan view of a fragment of a catalyst block encased in theface of the housing of FIGURE 1, where the face of the housing is in acircular shape.

FIGURE 4 is an elevational view of an embodiment of a catalytic heaterunit of this invention showing the catalyst block encased in the face ofa conical housing the primary air inspirators, the venturi, and theangle between the housing and a plane normal to the conic axis of thehousing.

FIGURE 5 is a side view of a slip ring which can be used to regulate theinflow of air at apertures 11.

FIGURE 6 is a top view of the slip ring of FIGURE 5.

DESCRIPTION OF PREFERED EMBODIMENTS 7 and even distribution of thecombustion mixture within the housing. The fuel and oxygen-containinggas is fed to the housing at the inlet port through a venturiarrangement. This combustible mixture may optionally be distributedthroughout the interior of the housing by use of an inverted cone, ascreen, or the like. The gaseous mixture is preferably fed axially intothe housing. Either air or the fuel may be fed to the venturi toinspirate the other. Air under pressure is preferably used to obtaincombustion mixtures having greater than twice the stochiometric amountof air.

It has been learned that when a conical housing is utilized, particularexpansion angles are preferred in order to provide the least amount ofirregular heating of the supported catalyst and prolong catalyst life.Expansion angle, as used herein, refers to the angle between the conicsurface and the normal to the conic axis in the plane containing theconic axis and its normal. An expansion angle of about 20-80 ispreferred.

The unitary ceramic block support used in connection with this inventionis located in the cross-sectional area of the face of the housing andmay occupy approximately all of this area. The ceramic may be used inmany configurations, e.g. round, square, conical, spherical, and it maybe raised or partially raised or recessed from the surface of thehousing.

The unitary inert refractory skeletal structure support for the catalystuseful according to this invention is characterized by having a largeplurality of unobstructed gas fiow channels or paths extendingtherethrough in the direction of gas flow. The gas flow channels can beone or more of a variety of cross-sectional shapes and sizes. Thechannels can be of the cross-sectional shape, for example, of the shapeof a trapezoid, rectangle, square, sinusoid, or circle so thatcross'sections of the support represent a 3 pattern that can bedescribed as a honeycomb, corrugated or lattice structure.

The meramic support is constructed of a substantially chemically andcatalytically inert, porous, rigid, solid refractory material capable ofmaintaining its shape and strength at high temperatures, for instance upto about 1,100 C. or more. It preferably has a low thermal coefficientof expansion which is less than 6 l per C. between 30 C. and 700 C.,this being of importance for good shock resistance. The refractorymaterial has a bulk density of about 0.4-2.0 grams per cubic centimeter,preferably about 0.51.5 grams per cubic centimeter and is unglazed andessentially entirely crystalline in form and marked by the absence ofany significant amount of glassy or amorphous matrices, for instance ofthe type found in poreclain materials. Further, the skeletal structurehas considerable accessible porosity as distinguished from thesubstantially non-porous porcelain utilized for electrical applications,for instance spark plug manufacture, characterized by having relativelylittle accessible porosity. The accessible pore volume not including thevolume of the gas flow channels is generally in excess of about 0.01cubic centimeter per gram of skeletal structure, preferably betweenabout 0.03 and 0.3 cc./g.

The walls of the channels of the unitary ceramic support structures usedaccording to this invention contain a multiplicity of surface macroporesin communication with the channels to provide a considerably increasedaccessible catalyst surface, and a substantial absence of small poresfor high temperature stability and strength. Whereas the superficialsurface area of such structures may be of the order of about 0.001 to0.01 m. /g. including the channels, the total surface area is typicallymany times greater, so that much of the catalytic reaction may takeplace in the large pores. Typically the total accessible surface area ofthe support is between about 0.1 and 2.0 m. /g. The skeletal structurehas a macropore distribution such that over 95% of the pore volume is inpores having a diameter greater than about 200 Angstrom units.

The total geometric or apparent surface area of the carrier includingthe walls of the gas flow channels will often be about 0.5 to 12,preferably 1 to 7 square meters per liter of support. The channelsthrough the ceramic support can be of any shape and size consistent withthe desired superficial surface and should be large enough to permitfree passage of the combustible gas mixture therethrough. The channelsmay be parallel or generally parallel and extend through the supportfrom one side to an opposite side, such openings being separated fromone another by preferably thin walls defining the openings.Alternatively, a network of channels may permeate the body to form atortuous gas flow path through the ceramic. The channel inlet openingsare distributed across the entire face or cross-section of the support.The preferred ceramic supports used in the catalytic heaters of thisinvention are of zircon-mullite, mullite, mullite-silica, alpha alumina,alumina-silica-magnesia and zirconium silicate. Examples of otherrefractory crystalline ceramic materials utilizable in place of thepreferred materials as support or carrier are sillimanite, magnesiumsilicates, zircon, petalite, spodumene, cordierite, andaluminasilicates.

Refractory metal oxide is deposited on the porous unitary ceramicsupport as a continuous thin deposit or as discontinuous thin depositspreferably of thickness of about 0.0004 to 0.001". This catalyticmaterial is a calcined refractory metal oxide which itself ischaracterized by a porous structure and which possesses a large internalpore volume and total surface area. Generally, the total surface area ofthe refractory metal oxide is at least about 10 square meters/ gram,preferably at least about 1-00 square meters/gram. Such oxides can beprepared by dehydrating preferably substantially completely the hydrateform of the oxide by calcination usually at temperatures of about 10 C.to 800 C. The preferred active metal oxides contain members of the gammaor activated alumina family which can be prepared, for instance, byprecipitating a hydrous alumina gel and thereafter drying and calciningto expel hydrated water and provide the active gamma-alumina. Aparticularly preferred active refractory metal oxide is obtained bydrying and calcining at temperatures of about 300 C. to 800 C. a mixtureof precursor hydrous alumina phases predominating in crystallinetrihydrate, that is, containing in excess of about 50% of the totalalumina hydrate composition, preferably about 65 of one or more of thetrihydrate fonns gibbsite, bayerite and nordstrandite by X-raydiffraction. The substantial balance of the hydrate, preferably about35% to 5%, may be amorphous hydrous or monohydrous boehmite alumina.Calcination of the precursor hydrous alumina is preferably controlled sothat the gamma-alumina obtained contains monohydrate alumina in anamount substantially equivalent to that originally present in themixture of the high trihydrate precursor hydrous alumina phases. Othersuitable catalytic refractory metal oxides include for example active orcalcined beryllia, Zirconia, magnesia, silica, etc., and combinations ofmetal oxides such as boria-alumina, silica-alumina, thoria-alumina, etc.Preferably the activated refractory oxide is composed predominantly ofoxides of one or more metals of Groups II, HI and IV having atomicnumbers not exceeding 40. The active refractory metal oxide deposit mayconstitute about 10 to 300 grams per liter of the ceramic support,preferably about 30 to grams per liter.

Depositing the active refractory metal oxide on the support may beaccomplished in several ways. One method involves dipping the supportinto a solution of the salt of the refractory metal and calcining todecompose the salt to the oxide form. A more preferred method comprisesdipping the support into an aqueous suspension, dispersion or slurry ofthe refractory oxide itself, drying and calcining. In this method,suspensions or dispersions having a solids content in range of about 10%to 70% by weight can be used to deposit a suitable amount of an activerefractory metal oxide on the support in a single application. In orderto prepare a catalyst having 10% activated alumina on a zircon-mullitestructure, about 20%40% solids in the suspension is used. The percentsolids is determined on an ignited weight basis (ignited at l,100 C.).In general, calcining temperatures Within the range of about C. to 800C. are employed. The Calcination is favorably conducted in air, forexample flowing dried air, or may be carried out in contact with othergases such as oxygen, nitrogen, hydrogen, flue gas, etc. or under vacuumconditions. The refractory oxide is deposited on the surfaces of theskeletal structure including the channel surfaces and the surfaces ofthe superficial macropores in communication with the channel surfaces asthin deposits in an amount, by weight, of about 1% to 50% and preferably5% to 30% based on the weight of the skeletal structure.

The gas flow channels of the unitary ceramic sup ported catalyst hereinare thin-walled channels providing a large amount of superfiicialsurface area. The walls of the cellular channels are generally of theminimum thickness necessary to provide a strong unitary body. This wallthickness will usually fall in the range of about 2 to 10 mils. withthis wall thickness the structures contain from about 252,500 or moregas or flow channel inlet openings per square inch and a correspondingnumber of the gas flow channels, preferably about 100'2,000 gas inletand flow channels per square inch. The open area should be in excess of60% of the total area. The size and dimensions of the ceramic supportuseful according to this invention can be varied widely as desired.

The catalyst supports providing the multitplicity of gas flow channelscan be prepared from any of the refractory ceramic materials previouslymentioned herein. One

method of preparing such supports is by applying by spraying, dipping orbrushing a suspension of the pulverized ceramic material and an organicbinder, for instance gum arabie, colophony, acrylate resins,methacrylate resins, alkyl resins, phenolic resins or a chlorinatedparafiih, to each side of a plurality of flexible organic carriersheets, for instance of cellulose, acetate paper, onion skin paper,nylon cloth or polyethylene film. Several of the thus-coated carriersheets are then corrugated by, for instance crimpling or multi-foldingthe sheets, and the remaining coated carrier sheets are left in theiroriginal fiat condition. The coated corrugated and fiat sheets are thensuperposed one on another in alternate corrugated and flat sheetrelationship. The resultant multi-layer structure is then fired in afurnace at a slow rate to prevent breakage due to thermal shock and to atemperature sufficiently high to sinter the ceramic particles into aunitary structure. During the firing the organic binders are removed bydecomposition and volatilization. Such preparation method is disclosedin BritishPatent 882,484. The porous inert unitary solid refractoryskeletal structure support having the plurality of gas flow channels isalso obtained in commerce from the Minnesota Mining and ManufacturingCompany who supplies the supports with 7 and 11 corrugations per linearinch. The platinum group metal, for example platinum or palladium, maybe deposited on the support so that the supported catalyst may containabout 0.1%l 0% metal and preferably 0.052% metal based on the totalweight of the catalyst plus support. The catalytic metal is preferablydeposited directly on the refractory metal oxide activated carrier.

Application of the catalytic metal to the ceramic support can beeffected by immersing the skeletal structure, preferably with therefractory metal oxide deposited thereon, in an aqueous solution of awater-soluble inorganic salt or salts of the particular metal or metals,followed by agitating the mixture and precipitating the metal or metalstypically in chemically combined state, for instance as oxides, on theskeletal structure. The metal oxide can then be reduced, when the metalform catalyst is desired, by contacting same with a reducing gas, e.g. Hat an elevated temperature of between about 100 and 1,l00 C.

Catalytic heaters having radiating catalyst surfaces of about 1 to 8inches in its greatest dimension are conveniently prepared althoughlarger units using a plurality of unitary supports may also be used. Formost purposes and to prevent the need for internal supports, dimensionsof about 2 to 4 inches are preferred. The depth of the honeycombcatalyst can be about one inch or more but depths of less than /2" andmore than are quite satisfactory. Depths of about A" have been foundparticularly satisfactory and are generally preferred.

The fuels used by the catalytic heaters of this invention may be a gas,a vapor, atomized liquid or the like and may comprise volatilehydrocarbons including straight chain hydrocarbons such as loweralkanes, e.g. ethane, propane, butane, heptane and the like. Otherusable fuels include aromatics such as benzene, cycloparafiins such ascyclohexane, acetylene, alcohols such as lower alkanols, e.g. methanol,ethanol, isopropanol, and the like.

These fuels may be mixed with air or oxygen-containing gas in a widerange of oxygen-fuel ratios. Generally, at least stoichiometric amountsof oxygen are preferred and amounts of 0.2 to 20 times thestoichiometric amount are also effectively utilized. Preferably, about1.05 to 3 times the stoichiometric amount of oxygen is desired. This isa particularly advantageous feature of this invention. An amount ofoxidant such as air may be used in accordance with the present inventionwhich would subject known catalytic heaters to flash-back when littleoxidant is used, and to flame lifting and extinction when large amountsof oxidant are used, e.g. 2 times the stoichiometric amount, especiallywhen the oxidizing surface is exposed to a slight cross-wind. Thepresent radiant heaters of the invention have been exposed to airvelocities of 40 mph. across the face of the ceramic block with noflash-back.

These novel and new radiant heaters generally are best started by usingless than the normal operational amounts of oxidant. This permits veryrapid increase in temperature of the cold catalyst. Once operatingtemperature is approached, the amount of oxidant may be adjusted toprovide the desired oxidant-fuel ratio.

The unique fiameless catalytic heater of this invention has still afurther desirable feature-that of being selfigniting when a combustiblemixture is supplied, even up to about four minutes after the heater hasbeen turned off. This is due to heat retention by the catalyst supportand the very high activity of the catalyst. This features makesutilization of this heater very simple and convenient duringapplications, requiring its intermittent use.

As earlier mentioned, the turn-down ratio of these heaters is verybroad, in the range of about :1. This is substantially greater than thatfor other flameless heaters. This feature permits use of a single heaterfor one application requiring heating with a device using only about 15B.t.u./in. as well as for a second application requiring heater capacityof 1,500 B.t.u./in.

Referring to the drawings: In FIGURE 1 the housing 2 has an inlet port 1and a face 3. Face 3 is an opening which has a ceramic catalyst blocktherein. This ceramic catalyst block fills virtually the entirecross-sectional area of the opening and accordingly generally conformsto the shape of this face. FIGURES 2 and 3 represent two of the variousshapes which the cross-sectional area of the housing and the ceramiccatalyst block therein may be. The ceramic catalyst block has parallelchannels 4 disposed in the direction of gas flow and has catalyst 5deposited thereon. The catalyst block may be retained in the housing byany suitable means. FIGURE 4 shows the ceramic catalyst block 8 held inplace in the face of the housing 7 by ring 9, which is biased againstthe inner wall of the housing. Apertures 10 and 11 are sources of airinspiration into the housing. Venturi 6 is the port of entry for thefuel. Either fuel or air may be fed to the venturi 6 to inspirate theother. Pin 12 is used for temporarily retaining slip ring 13 shown inFIGURES 5 and 6, in order to limit the inflow of air through apertures11.

In operation of the embodiment shown in FIGURE 4, for example, a gaseousfuel, e.g. propane, under pressure is fed to venturi 6 causinginspiration of the oxidant gas, e.g. air. The fuel and air, mixed in theventuri, pass into the housing 7 and ignite at the ceramic catalystblock 8. Additional air is inspirated through apertures 11. Whenigniting a cold burner, slip ring 13 is preferably mounted on pin 12 inorder to limit inflow of air through apertures 11.

It should be understood that although this invention has been describedwith reference to particular embodiments thereof, changes andmodifications may be made which are within its intended scope, and itshould be limited only by the language of the appended claims.

What is claimed is:

1. A catalytic heater unit for supplying radiant heat having a turn-downratio of about 100 to 1, and capable of using 0.2 to 20 times thestoichiornetric amount of oxygen comprising:

(a) a housing having a gas inlet port and a face,

(b) venturi means adjacent to the gas inlet port for supplying a mixtureof fuel and an oxygen-containing gas into the housing,

(0) a catalytic burner-radiator block mounted within the face comprising(1) a unitary ceramic block support with channels directing the gas flowtherethrough, said ceramic block having an open area in excess of 60% ofthe total area,

(2) a refractory metal oxide having a total surface area of at leastabout square meters/ gram deposited on the support, and

(3) a platinum group metal deposited on the refractory metal oxidewhereby said catalytic burner block permits efficient combustion, readyself ignition of the fuel, and resistance to flashback.

2. A catalytic heater unit according to claim 1 wherein the unitaryceramic block support contains 252,500 flow channel inlet openings persquare inch.

3. A catalytic heater unit according to claim 2 wherein the refractorymetal oxide is alumina and the platinum group metal is platinum.

4. A catalytic heater unit according to claim 3 wherein the catalyticburner-radiator block is comprised of a unitary ceramic block support ofabout 1 to 8 inches in its greatest dimension and A to 1 inch in depth,whereby the block permits ready self-ignition of the fuel and resistanceto flash-back.

5. A catalytic heater unit according to claim 1 wherein the housing isof generally frusto-conical configuration.

6. A catalytic heater unit according to claim 5 wherein the expansionangle of the housing is about 20 to 7. A catalytic heater unit accordingto claim 6 wherein the platinum isdeposited to the extent of 0.05 to 10%by Weight based on the total Weight of the catalyst plus support.

8. A catalytic heater unit according to claim 1 having means forrestricting the flow of oxygen into the housing during initial ignition.

9. A catalytic heater unit according to claim 1 wherein the refractorymetal oxide deposit has a surface area of at least about square meters/gram.

References Cited JAMES W. WESTHAVER, Primary Examiner.

